Note: Most virus scan products use a file system filter driver. The only way to completely "disable" virus scan is to uninstall it and reboot the computer.

Resolution

In one case, virus scan software was out of date by several months. The fix was to update the virus scan software and download the latest virus definition file

In another case that affected several locations, virus definitions had been automatically updated earlier in the day, and the new virus definitions caused the performance issue. The issue cleared when another virus definition update was published several days later